1. Field of the Invention
The present invention relates to a tonal gradation conversion of an image to be displayed on a display.
2. Description of the Related Art
Tonal gradation conversion is one of basic functions of an image display apparatus. The objective of the image display apparatus is to exactly convey image information to a viewer. The tonal gradation characteristics of the image display apparatus, namely, a relationship between an input signal intensity “X” and an illumination “L”, is referred to as a gamma value “γ”, and expressed in the following equation 1.L=Xγ  [Equation 1]
According to studies, sensitivity of the human eye to luminance is roughly expressed as in equation 2, where “S” represents human eye sensitivity and “L” represents the intensity of light reaching the human eyes, and “a” is an index ranging from 0.3 to 0.4.S=Lα  [Equation 2]
In accordance with equations 1 and 2, the input signal intensity X and the sensitivity S are substantially proportional to each other. The gamma value γ of the display apparatus is defined in this way.
In a display apparatus, a gamma value γ of 1.8 is typically used so that printing results and the output of the display apparatus appear the same to the human eyes.
Standardization of the display apparatuses has been conventionally performed by measuring luminance characteristics of the display and then by adjusting the gamma value γ. The standardization allows displayed images to remain substantially the same from viewer to viewer without regard to difference between display apparatuses.
However, the object of the display apparatus is to convey exact image information to a viewer rather than standardization information of the display apparatus itself. Further, each viewer has his or her own characteristics, and the value of a in equation 2 is not always the same for all individuals. The characteristics expressed by equation 2 are not always equally applicable to all individuals.
Despite standardization, content of recognizable image information stored in the computer potentially appears different from viewer to viewer if the display apparatus is adjusted based on the standardization alone because such standardization does not take into consideration the characteristics of each viewer.
In particular, this effect is pronounced in the field of medical image display apparatuses and doctors can have difficulty learning the location of an injury and illness on a medical image.
Japanese Patent-Laid Open No. 2001-34255 discloses a method and an apparatus for selectively using one from a plurality tonal gradation conversion tables to compensate for a degradation in luminance of a display device. One disadvantage of this reference is that it fails to select the table based on the vision characteristics of the viewer.